1. Field of the Invention
The present invention relates to a temperature control method for use in a fixing device that fixes a toner image on a recording sheet by heating the recording sheet and an image forming apparatus incorporating such a temperature control method.
2. Discussion of the Background
In an image forming apparatus such as a copying machine, a printer, a facsimile machine or a multifunction device incorporating several of these functions, a fixing device for melting a toner image by heat and fixing it on a recording medium such as a sheet for printing or the like is often employed. FIG. 1 shows an example of a configuration of the fixing device.
As shown in FIG. 1, a fixing device 100 includes a fixing roller 110, a heating roller 120, a fixing belt 130, a pressure roller 140 and the like. The fixing belt 130 is extended between the fixing roller 110 and the heating roller 120. The heating roller 120 has a heater 150 inside. By having the heater 150 generate heat so as to heat the heating roller 120, the fixing belt 130 is also heated. Also, the pressure roller 140 is pressed against the fixing belt 130 at a position opposing the fixing roller 110 to form a fixing nip N therebetween. When a printing sheet P on which a toner image T has been transferred passes through the fixing nip N, the toner is melted and the image is fixed on the printing sheet P.
In order to ensure that the fixing device performs reliably, a temperature of the fixing belt must be maintained at a target temperature set in advance. Thus, a temperature detector 160 for detecting a temperature of the fixing belt 130 is disposed as shown in FIG. 1 for temperature control of the fixing belt 130.
As a temperature control method for the fixing belt, for example, an ON/OFF control method in which the heater 150 is turned on/off according to the temperature of the fixing belt 130 as measured by the temperature detector 160 is known. Specifically, if the temperature of the fixing belt is lower than a target temperature, a heater is turned on, while if the temperature of the fixing belt is higher than the target temperature, the heater is turned off.
However, employing only the ON/OFF temperature control method, the temperature of the fixing belt might deviate substantially from the target temperature. In order to decrease a temperature difference (also referred to as a temperature ripple) between the temperature of the fixing belt and the target temperature, an image forming apparatus shown in Japanese Unexamined Patent Application Publication No. 2006-323093, for example, executes PID control. PID control is a control method for optimizing a plurality of parameters according to a deviation between a detected temperature and a target temperature by combining proportional, integral, and differential with a control algorithm.
PID control is described referring to FIG. 2.
As an initial matter, if the temperature difference between a temperature T1 of a fixing belt and a target temperature T0 is large (e.g., 100 degrees or more), a heater duty D of a heater is increased for heat generation (proportional control). Thereafter, when the temperature T1 of the fixing belt approaches the target temperature T0, the heater duty D of the heater is decreased (differential control) so that the temperature T1 of the fixing belt does not exceed (overshoot) the target temperature T0. Then, in order to eliminate the difference between temperature T1 of the fixing belt and the target temperature T0, the heater duty D is adjusted (integral control).
When a toner image is fixed onto a printing sheet at a fixing nip, since the printing sheet draws heat from the fixing belt the temperature of the fixing belt decreases. At this time, in order to raise the lowered temperature of the fixing belt to a target temperature, a heater is caused to generate heat. However, it takes time for the heat generated by the heater to raise the temperature of the fixing belt, and as a result, the temperature of the fixing belt may not be maintained at an appropriate temperature and proper fixing might not occur.
Therefore, Japanese Patent No. 3216386, for example, discloses a temperature control method that compensates for heat drawn off by a printing sheet by electrifying a heater in advance, that is, before the printing sheet enters a fixing nip. Accordingly, responsiveness of the temperature control of a fixing belt is improved, and image quality is stabled.
However, a problem with the PID temperature control method described above is that, if the measured temperature of the fixing belt and the target temperature are close to each other, it is not possible to greatly increase the heater duty for heating. Consequently, increase of the heater duty is gentle even if the heater is electrified in advance before the entry. As a result, when the printing sheet enters the fixing nip when the temperature of the fixing belt and the target temperature are close to each other, the temperature of the fixing belt is rapidly lowered.
This phenomenon is illustrated in the graph shown in FIG. 2, which shows a temperature of the fixing belt, a target temperature of the fixing belt, and heater duty of a heater in the case of temperature control of the fixing belt by the related-art PID control. In FIG. 2, the area below the line T0 (target temperature) but above the line T1 (actual measured temperature) and indicated by the asterisk (*) is the shortfall created between the target temperature of the belt and the actual temperature of the belt due to this flaw in the PID temperature control method.